Releasable mill

ABSTRACT

A mill assembly having a milling head which is releasable from the mill body, such as by shearing a shear pin. The release mechanism can be released by dropping a pumpable plug through the work string to block fluid flow through the releasable milling head, or by increasing fluid flow through a constriction in the releasable milling head to increase the back pressure above the milling head. A check valve in the milling head can prevent uphole flow through the work string in the event of a pressure excursion. A fishing neck can be attached to the milling head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U. S. patent application Ser. No. 10/916,773, filed Aug. 11, 2004, for “Releasable Mill”, which claims the benefit of U. S. Provisional Patent Application No. 60/495,021, filed Aug. 13, 2003, for “Releasable Bridge Plug Mill”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of apparatus used to mill out downhole equipment in a well, such as in an oil or gas well.

2. Background Art

Some oil or gas wells are drilled into locations at which multiple oil or gas formations are found, at different depths. That is, one hydrocarbon formation may be above or below another, and there may be more than two such formations at different depths. It is common to produce hydrocarbons from only one selected formation at a time. One means used to assist in this type of production is a plug, which can be installed in the bore hole or casing, between two of the formations. Such a plug isolates one formation from another, while allowing access to the upper formation via the bore hole. It is also common to remove such a plug, in order to allow access to the lower formation, via the bore hole, for the purpose of producing hydrocarbons, or for other purposes.

When such a plug is removed, it is often removed by lowering a mill into the bore hole or casing, attached to a work string. The mill is usually provided with some type of cutting structure on its lower face, and this cutting structure is often dressed with some type of cutting material, such as inserts or abrasives. The mill is lowered into contact with the upper end of the plug; then, the work string is rotated, thereby rotating the mill. Alternatively, a downhole motor can be used on the work string, as is commonly known in the art, and the mill can be rotated by operating the downhole motor. In either case, as the mill is rotated, the cutting structure cuts the plug into small cuttings, which are returned to the surface entrained in the drilling fluid which is pumped downhole through the work string. This operation is continued until the entire plug is removed, or until a sufficient portion of the plug is removed to allow the remaining portion to fall farther into the borehole.

After this type of operation, it is necessary to remove the mill from the bore hole before access to the lower formation is available. This is because, although the mill may have passageways for drilling fluid, these fluid passageways are not sufficiently large to provide the desired degree of access to the lower formation. The mill body itself is typically a substantially solid, comparatively hard, metal body. Therefore, in order to complete the operation, the work string and the mill must be pulled from the bore hole to provide the desired access to the lower formation. As is well known, tripping a work string into or out of a well is a time consuming, expensive process. It is desirable to have a method and apparatus for removing such plugs, or other types of objects in a well bore, while eliminating the necessity for tripping the work string out of the bore hole, to remove the mill and provide access to the lower formation.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a mill assembly having a releasable milling head attached to a mill body with one or more shear pins, or another releasable fastening feature. The mill assembly can be lowered into a bore hole to mill out a plug, after which the milling head can be completely released from the work string, such as by shearing the shear pins, and allowed to drop into the bore hole. Separation of the milling head from the mill body leaves a substantially open bore into and through the work string. The mill body and the work string can be left in the bore hole while production from the lower formation takes place, through this open bore. The milling head is provided with a check valve in the fluid path, to allow the downhole flow of drilling fluid during milling, but to prevent the uphole flow of fluids during a kick or pressure excursion. A fishing neck can also be provided on the milling head, to assist in the later removal of the milling head where desired.

The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a longitudinal section view of a first embodiment of the apparatus of the present invention;

FIG. 2 is a lower end view of the milling head portion of the apparatus shown in FIG. 1, and showing the location of the line along which the section in FIG. 1 is taken;

FIG. 3 is an upper end view of the milling head portion of the apparatus shown in FIG. 1;

FIG. 4 is a lower end view of the mill body portion of the apparatus shown in FIG. 1;

FIG. 5 is a longitudinal section view of the apparatus shown in FIG. 1, after complete separation of the milling head from the mill body;

FIG. 6 is a longitudinal section view of a second embodiment of the milling head of the present invention, with a ball check valve;

FIG. 7 is an expanded longitudinal section view of a third embodiment of the apparatus of the present invention, with a flapper check valve and a fishing neck;

FIG. 8 is an assembled longitudinal section view of the apparatus shown in FIG. 7;

FIG. 9 is a longitudinal section view of a ball clutch and fishing neck for use in a fourth embodiment of the apparatus of the present invention;

FIG. 10 is a longitudinal section view of a collet for use in the fourth embodiment of the apparatus of the present invention, along with the ball clutch and fishing neck shown in FIG. 9; and

FIG. 11 is an assembled longitudinal section view of the fourth embodiment of the apparatus of the present invention, incorporating the ball clutch and fishing neck, and the collet, shown in FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the mill assembly apparatus 10 of the present invention principally includes a mill body 12, to which a milling head 14 is releasably attached, such as by one or more shear screws or pins 16. The mill body 12 is adapted to be mounted on a work string (not shown) as is commonly known in the art, such as by threading thereto. A plurality of cutting inserts 18 can be provided on the lower face 34 of the milling head 14 to form a cutting structure. Alternatively, the cutting structure can include milled teeth, crushed carbide, or abrasives, without departing from the spirit of the present invention.

One or more torque lugs 20, better shown in FIG. 3, can be provided on an upwardly facing annular shoulder 46 of the milling head 14. These torque lugs 20 can extend into one or more torque notches 28, better shown in FIG. 4, formed on the lower end 48 of the mill body 12. An axially oriented inner face or shoulder 42 in each torque notch 28 abuts an axially oriented outer face or shoulder 40 on each torque lug 20. Rather than torque lugs and notches, mating shoulders could alternatively be used. When the milling head 14 is mounted to the mill body 12, the upwardly facing annular shoulder 46 of the milling head 14 abuts the lower end 48 of the mill body 12. Also, the upper end 36 of the milling head 14 can abut a downwardly facing annular shoulder 38 within the mill body 12.

The section shown in FIG. 1 is taken along a broken section line as shown in FIG. 2, to better illustrate a possible placement of the torque lugs 20 and torque notches 28, and the shear pins 16.

A fluid flow path can be provided through the mill body 12 and the milling head 14, which can for example include the inner bore 44 in the mill body 12, and a first conical surface 50, a ball seat 30, an inner bore 32, a second conical surface 52, an axial jet 24, and a plurality of angled jets 26 on the milling head 14. Drilling or milling fluid can be pumped down the work string (not shown) to flow through this fluid path in the mill body 12 and the milling head 14, as indicated by the arrows. In addition to the mill assembly apparatus 10, a pumpable ball or plug 22 can be provided for selectively restricting this fluid flow, as will be described below.

The mill assembly apparatus 10, assembled as shown in FIG. 1, is mounted to a work string (not shown) and lowered into a well bore, until the cutting structure on the lower face 34 of the milling head 14 contacts a plug or other item to be milled out of the bore hole. A rotatable work string or a downhole motor can be used, without departing from the spirit of the present invention. After contacting the plug to be milled, the mill body 12 is rotated in the clockwise direction, as viewed from the upper end, rotating the milling head 14 by virtue of the abutment of the axially oriented torque shoulders 40 and 42, and causing the inserts 18 or other cutting structure to mill the plug away. Cuttings or fragments of the milled plug are removed from the bore hole entrained in the milling fluid which is pumped through the mill body 12 and the milling head 14 and returned up the annulus to the surface.

After the plug has been milled away, the pumpable plug or ball 22 can be pumped downhole through the work string to land in the ball seat 30 in the milling head 14. Alternatively, the fluid flow rate can simply be increased through the apparatus 10 by increasing the speed of the fluid pumps. Either action results in an increased hydraulic pressure at a location in the fluid flow path as it passes through the milling head. If the pumpable ball 22 is used, the increased hydraulic pressure occurs primarily on the first conical surface 50 and across the top of the ball 22. If the increased pump speed is used, the increased hydraulic pressure occurs in the fluid flow path 50, 30, 32, 52, 24, 26. This increased hydraulic pressure exerts an increased downward hydraulic force on the upwardly facing components of the surfaces of the milling head 14 which are exposed to the increased pressure. As this downward hydraulic force reaches a sufficient, predetermined, level, it causes the shear pins 16 to shear.

When the shear pins 16 shear, the milling head 14 is completely released from the mill body 12 and completely separates therefrom, as shown in FIG. 5. This complete axial separation of the milling head 14 from the mill body 12 allows the milling head 14 to fall downhole, completely opening up the borehole at the previously plugged location. Since the torque shoulders 40 and 42 are axially oriented, they are adapted to separate from each other easily when the shear pins 16 shear, and they do not interfere with the shearing of the pins 16 or the complete axial separation of the milling head 14 from the mill body 12.

After complete separation of the milling head 14 from the mill body 12, the inner bore of the mill body 12 is completely open to allow for flow of hydrocarbon fluids upwardly through the mill body 12 as shown by the arrows in FIG. 5. The separated mill body 12 thus performs thereafter as simply an extension of the work string, and the hydrocarbon fluid flow continues upwardly through the work string to the surface. Therefore, the complete separation of the milling head 14 from the mill body 12 allows for the efficient production of hydrocarbons from the bore hole, through the work string, without pulling and replacing the work string with a production tube.

A second embodiment of the milling head is shown in FIG. 6. This embodiment of the milling head 140 can be fitted with a check valve comprising a ball seat 142, a check ball 144, and a spring 146. It can be seen that, as milling fluid passes downhole through the fluid path in the milling head 140, the check ball 144 can be lifted off its seat 142, against the bias of the spring 146, to allow flow out the lower end of the work string. A kick or pressure excursion sometimes occurs in the formation fluids, which could create an undesirable flow in the uphole direction through the work string. To prevent this, the spring 146 biases the check ball 144 toward engagement with its seat 142. As pressure below the milling head 140 increases above the drilling fluid pressure, this causes the check ball 144 to seat more securely, thereby preventing flow in the uphole direction.

A third embodiment of the apparatus 210 of the present invention is shown in FIGS. 7 and 8. In this embodiment, the mill body 212 is secured to the milling head 214 by shear pins 216 in shear pin bores 224 and 226 in the mill body 212 and the milling head 214, respectively. Flow passages 228 are provided through the milling head 214. However, in this embodiment, the check valve comprises a swing check type valve, with a check valve body 262 assembled in the milling head 214, and with a flapper valve 264, which is pivotably mounted to the check valve body 262 by a pivot pin 266. The check valve body 262 can be retained in the milling head 214 by one or more snap rings or pins, as is known in the art. The flapper valve 264 is biased toward the closed position by a spring. Flow of fluid down through the apparatus can open the flapper valve 264 against the spring bias, but backflow through the check valve is prevented by shutting of the flapper valve 264, which seats against the lower side of the check valve body 262.

Also provided in this embodiment is a fishing neck 260, which is retained in the milling head 214, above the check valve body 262, by one or more snap rings or pins, as is known in the art. A ball seat 230 is provided in the upper side of the check valve body 262. When milling has been completed, and it is desired to release the milling head 214 from the mill body 212, a ball 222 is pumped downhole through the work string, to seat in the ball seat 230. Increasing pressure above the pumpable ball 222 then shears the shear pins 216, releasing the milling head 214 from the mill body 212, as in the first embodiment. If it is desired to subsequently remove the milling head 214 from the well bore, known fishing techniques can be used to attach to the fishing neck 260 and pull the milling head 214.

A fourth embodiment of the apparatus 310 of the present invention is shown in FIGS. 9, 10, and 11. In this embodiment, a ball clutch mechanism 360 is provided, incorporating a fishing neck, and including one or more ball clutch bores 324 through the wall of the ball clutch 360. One or more clutch balls 326 are positioned in the clutch bores 324, when the ball clutch 360 is assembled to the mill body 312. The clutch balls 326 are forced outwardly in an inner annular groove within the mill body 312 by a collet 370 which is positioned in the inner bore of the ball clutch 360. The fingers on the upper end of the collet 370 are outwardly biased to seat in an inner shoulder of the fishing neck and ball clutch 360. This positioning of the collet 370 releasably retains the ball clutch 360 to the mill body 312. The ball clutch 360 is, in turn, secured to the milling head 314 by one or more snap rings or pins, as is known in the art. So, the ball clutch mechanism 360 releasably retains the milling head 314 to the mill body 312.

Flow passages 328 are provided through the milling head 314. This embodiment of the apparatus 310 can be fitted with a check valve comprising a ball seat 340 in the lower end of the ball clutch 260, a check ball 344, and a spring 346. It can be seen that, as milling fluid passes downhole through the fluid path in the milling head 314, the check ball 344 can be lifted off its seat 340, against the bias of the spring 346, to allow flow out the lower end of the work string. To prevent a kick or pressure excursion, the spring 346 biases the check ball 344 toward engagement with its seat 340. As pressure below the milling head 314 increases above the drilling fluid pressure, this causes the check ball 344 to seat more securely, thereby preventing flow in the uphole direction.

A ball seat 330 is provided in the upper side of the collet 370. When milling has been completed, and it is desired to release the milling head 314 from the mill body 312, a ball 322 is pumped downhole through the work string, to seat in the ball seat 330. Increasing pressure above the pumpable ball 322 then forces the collet fingers inwardly, releasing the collet 370 from the inner shoulder in the ball clutch 360. After the collet 370 is released in this fashion, it is forced further downwardly by fluid pressure. This downward movement of the collet 370 allows the clutch balls 326 to be released from the inner groove in the mill body 312, releasing the ball clutch 360 and the milling head 314 from the mill body 312. If it is desired to subsequently remove the milling head 314 from the well bore, known fishing techniques can be used to attach to the fishing neck on the ball clutch 360 and pull the milling head 314.

While the particular invention as herein disclosed is capable of obtaining the objects hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention. 

1. A milling head assembly for use on a work string, comprising: a milling head; at least one cutting structure on said milling head; a fluid path through said milling head; a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path; a fishing neck attached to said milling head; and a releasable fastening mechanism fixedly attached to one of said milling head or a work string, said fastening mechanism being adapted to attach said milling head to a work string, said fastening mechanism being adapted to selectively release said milling head from a work string, thereby completely separating said milling head from a work string.
 2. The milling head assembly recited in claim 1, further comprising a torque transfer device on said milling head, said torque transfer device being adapted to transfer torque from a work string to said milling head, said torque transfer device being adapted to allow said separation of said milling head from a work string.
 3. The milling head assembly recited in claim 1, further comprising a flow restricting device, said flow restricting device being adapted to impede fluid flow through said fluid path at a location on said milling head, thereby creating a predetermined hydraulic force on said milling head to release said fastening mechanism.
 4. The milling head assembly recited in claim 1, wherein said check valve is a ball check valve.
 5. The milling head assembly recited in claim 1, wherein said check valve is a flapper check valve.
 6. A mill assembly for use on a work string, comprising: a mill body adapted to be attached to a work string and lowered into a well bore; a milling head; at least one cutting structure on said milling head; a fluid path through said mill body and said milling head; a check valve in said fluid path of said milling head, oriented to prevent fluid flow in the uphole direction through said fluid path; a fishing neck attached to said milling head; and a releasable fastening mechanism fixedly attached to one of said milling head or said mill body, said fastening mechanism being adapted to attach said milling head to said mill body, said fastening mechanism being adapted to selectively release said milling head from said mill body at a predetermined hydraulic force on said milling head, thereby completely separating said milling head from said mill body.
 7. The mill assembly recited in claim 6, further comprising a torque transfer device on said milling head, said torque transfer device being adapted to transfer torque from said mill body to said milling head, said torque transfer device being adapted to allow said complete separation of said milling head from said mill body.
 8. The mill assembly recited in claim 6, further comprising a flow restricting device, said flow restricting device being adapted to impede fluid flow through said fluid path at a location on said milling head, thereby creating said predetermined hydraulic force on said milling head.
 9. The milling head assembly recited in claim 6, wherein said check valve is a ball check valve.
 10. The milling head assembly recited in claim 6, wherein said check valve is a flapper check valve.
 11. A method for milling an object in a well bore, comprising: providing a mill body attached to a work string and a milling head mounted to said mill body with a releasable fastening mechanism, said milling head having a cutting structure and a fishing neck; lowering said mill body and milling head into a well bore on the work string; rotating said mill body and milling head to mill an object in the well bore with said cutting structure; preventing any fluid flow in the uphole direction through said fluid path; releasing said releasable fastening mechanism to completely release said milling head from said mill body; and retrieving said milling head from the well bore by attaching to said fishing neck.
 12. The method recited in claim 11, further comprising: providing a separate torque transfer device on said milling head; transferring torque from said mill body to said milling head with said torque transfer device; and disengaging said torque transfer device upon said complete separation of said milling head from said mill body.
 13. The method recited in claim 11, further comprising impeding fluid flow at a location on said milling head, to increase hydraulic pressure and release said releasable fastening mechanism. 